Amstrad Cartridge Identification Device

ACID protection chip within a cartridge (bottom)

The ACID (Amstrad Cartridge Identification Device) protection chip is used in the cartridges of the Plus series and the GX4000 game console.

Technical

The 16pin ACID chip (in the cartridge) is sending a serial data stream to the 160pin ASIC chip (in the computer). The data stream is generated by some shift/xor logic, and is also affected by the EPROM address/enable signals. If the data stream isn't correct, then the ASIC does scatter access to RAM, making the cartridge unusable.

;ACID reverse-engineered 13-16 February 2010 by nocash (Martin Korth)
;below is repeated on every CLK cycle...
CmpVal=13596h, XorVal=0c820h
if PinA0=1 then CmpVal=CmpVal XOR 0000ch, XorVal=XorVal XOR 00004h
if PinA1=1 then CmpVal=CmpVal XOR 06000h, XorVal=XorVal XOR 06000h
if PinA2=1 then CmpVal=CmpVal XOR 000c0h, XorVal=XorVal XOR 00080h
if PinA3=1 then CmpVal=CmpVal XOR 00030h, XorVal=XorVal XOR 00020h
if PinA4=1 then CmpVal=CmpVal XOR 18000h, XorVal=XorVal XOR 08000h
if PinA5=1 then CmpVal=CmpVal XOR 00003h, XorVal=XorVal XOR 00000h
if PinA6=1 then CmpVal=CmpVal XOR 00600h, XorVal=XorVal XOR 00000h
if PinA7=1 then CmpVal=CmpVal XOR 01800h, XorVal=XorVal XOR 00800h
if PinCE=0 AND (ShiftReg OR 100h)=CmpVal then ShiftReg=ShiftReg XOR XorVal
NewBit=ShiftRegBit0 XOR ShiftRegBit9 XOR ShiftRegBit12 XOR ShiftRegBit16
ShiftReg=(ShiftReg SHR 1) + (NewBit SHL 16)
Wait for falling edge on PinCLK
if PinCCLR=0 then ShiftReg=1FFFFh      ;\done at falling CLK edge
PinSIN=ShiftRegBit0                    ;/

Replacing or overwriting the EPROM in existing cartridges is easy, so the ACID doesn't prevent piracy or homebrew programming. However, it prevents commercial developers from producing unlicensed cartridges.

Pinout

           +--()--+
       GND |1   16| +5V      AMSTRAD 40908
       A7  |2   15| A6       1L03P1003
       CLK4|3   14| A5       JAPAN
       NC  |4   13| A4
       CCLR|5   12| A3
       SIN |6   11| A2
       /CE |7   10| A1
       GND |8    9| A0
           +------+

Pin number	Name	Direction	Description
1	GND	-	Ground
2	A7	Input	EPROM Address A7
3	CLK4	Input	Clock (4 MHz)
4	NC	-	Not connected
5	CCLR	Input	Reset ACID
6	SIN	Output	Serial Data
7	/CE	Input	EPROM Chip enable
8	GND	-	Ground
9	A0	Input	EPROM Address A0
10	A1	Input	EPROM Address A1
11	A2	Input	EPROM Address A2
12	A3	Input	EPROM Address A3
13	A4	Input	EPROM Address A4
14	A5	Input	EPROM Address A5
15	A6	Input	EPROM Address A6
16	+5V	-	Vcc

Note: The two GND pins are interconnected with each other (inside of the chip). The NC pin seems to be always high.

Misc

• Test Circuit used for ACID reverse engineering

Investigations

• http://amstradcpc.mforos.com/305097/7723493-que-hace-exactamente-el-chip-acid-de-los-cartuchos/ (Spanish language)

• Vieuxzordiland's page on the matter. In French

• The CPCwiki's forum thread featuring the Epic fight between the infamous ACID and NoCa$h.

The End of the Darkness : A New Hope

At first was an age of Grim Darkness, Ignorance and Despair.

For twenty years, the once glorious Amstradom struggled and was about to loose it to the might of the Commodore's legions and Nintendorks hordes, a fight It wouldn't win.

Remembering with Nostalgia old ages of glory, gay old time when the Amstradom was young and fool, as told by the Elders. Until one day the end of a Nightmarish Time was about to End as announced by an obscure and lost prophecy.

A Lone Man came to stand before the Foul Darkness of Ignorance.

"Give me two days, and I'll bring you the light, I'll bring you hopes, for it is our destiny to stand proudly and face bravely." he said to the massed crowd of the Amstradom

The Amstrad Peoples frowned in horror and fear.

"Heresy, you know that None can stand before the ultime Might and Eldricht Arcane of the Mighty ACID, bringer of extinction, destroyer of the Worlds!" scanded the crowd in anger and fury.

"You fool, we are Doomed into Oblivion, the Commodore's Legions and Nintendork Hordes shall rule us all..." cried others.

"Burn this Foul Heretic for giving us false hopes ! We must accept our fate as we always did !" a last man hystericaly screamed.

Then Fano the Wise accomplished his destiny and stepped one pace before the crowd.

"I believe in you, Lone man, accept my humble sacrifice" he said gently, a tear in the eye.

For era we will remember Him who who sacrified his beloved Cartridge for the cause.

Strenghtened by the Blood of the martyr, the Lone Man Hurled himself into the fray.

Four days and Four nights, he fought.

The Battle was Epic, sounds of the bits tearing the sky, Heaven and Hell colour clashed in a furious destruction of blind madness.

Four days and Four nights, he Braved. So Grand was the cause and so inspiring was His Fight.

At first sceptic, the Amstradom were slowly gained to the Holy Cause of the Lone Man.

Fortified by the exemple of He who fought desperately yet with no hesitation, conscious that Grand Scheme was being at work.

Four days and Four night.

and then the Colour Clash ended.

The chanting cheerfull acclaims of Amstradom stand before the Lone Man.

Accompanied by colourfull sprites, smooth scrolling rolling as DMA channels rejoyced loudly, in a rainbow of four-thouzand-and-ninety-six Colours out of Space..

Victorious and Untainted.

He who defeated the Vile Beast.

"Four Days" he said.

"Fours days and Four nights, the Task was even greater than I could believe. How Fool was I... but I did overcame."

And future generations in Amstradom would remember Him as the Saviour, the Messiah, the Bringer of Light against Darkness.

He who shown us the way.

And NoCa$h was his name, the man who degfeated the ACID chip.

Twenty years and Four days.

A new Era was to begin. '